Wireless uplink sounding is considered to be useful for uplink (UL) and downlink (DL) scheduling, especially with channel reciprocity in time division duplex (TDD) mode systems. In the 3rd Generation Partnership Project (3GPP) standardization work groups, it has been proposed that the minimum sounding reference signal (SRS) bandwidth (BW) value be 4 resource blocks (RBs) and that all SRS bandwidths be multiples of 4 RBs. It has also been proposed that SRS bandwidths be tree-based. These assumptions help limit signaling requirements but may lead to missing out some edge resource blocks due to restricted flexibility. For example, for system BW of 81-110 RBs, the possible SRS BW is 4, 8, 12, 16, 20, 24, 32, 40, 48, 60, 64, 72, 80 and 96 resource blocks. The maximum number of unsounded resource blocks is 15, which corresponds to a maximum unsounded system BW of 2.7 MHz. This may be problematic in frequency selective channels. For example, a typical urban channel has a coherent channel bandwidth of roughly 200 KHz, which is narrower than the maximum unsounded system BW. The downlink scheduler also cannot simply assign the edge RBs for a distributed virtual resource block (DVRB), since mirroring is not used for DVRB operation.
In 3GPP, the durations and usage of an uplink pilot time slot (UpPTS) was proposed for TDD mode. If the UpPTS has a single (1) SC-FDMA symbol duration, it is used for sounding reference signals only. If the UpPTS has a two (2) OFDM symbol duration, it is used for short random access and/or SRS. The short random access, if present, may occupy from 1 to up to 6 groups of 6 RBs each, from the top or bottom of the band in alternating UpPTSs. Since there is no physical uplink control channel (PUCCH) or data in the UpPTS, problems that may occur when the SRS is transmitted in ordinary uplink sub-frames are not present when the SRS is transmitted in UpPTS. It is therefore considered to be beneficial for TDD to allocate SRSs in UpPTS rather than in ordinary uplink sub-frames. However, the unsounded RBs as explained above remain an issue. Moreover, with the special property of UpPTS, any unsounded RBs in UpPTS cannot be used easily for another purpose.
While there may be some benefits for full bandwidth sounding for UpPTS, the impact of deviating from current operation must be carefully considered. One consideration is that the sounding sequences should still be based on the existing UL demodulation reference signal (DM RS) sequences so that new sequences do not need to be generated and specified. Besides, the commonality of frequency division duplex (FDD) and TDD modes should be maximized for the benefits of module reuse.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.